Many machines such as engines, motors, compressors and the like, are connected to suitable supports via intermediate mounts. An hydraulic example of such a mount may be seen in FIG. 1. Engine 23 is supported by bracket 11 and working bushing 3. The bracket 11 is further connected, via the working bushing 3, to hydraulic mount 5. Hydraulic mount 5 is mounted onto a chassis 9 via a mount bracket 7. Such mounts are intended to isolate vibration, but must also be capable of supporting the weight of machine, engine, or motor and the like, and damping low-frequency forces of the machine relative to the support. These motions are caused by normal operations including variations in engine speed, load torque reaction etc. The design of such mounts is largely dependent upon the nature and types of forces transmitted between the machine and the support. In some applications such as gas-powered automobile engines, the mount may simply be an elastomeric block.
In other cases, such as a diesel engine, the mount may take the form of a spring and damper arranged in parallel with one another. There is unwanted forced transmissability in passive mounts due to the mount resonance and in the case of a parallel spring and damper, the damper inadvertently acts as an unwanted force transmitter at higher frequencies.
To overcome this problem, it has been proposed to add "active" elements to such machine mounts. Theoretically, such elements can be selectively controlled so as to effectively cancel the net dynamic forces transmitted through the spring and damper due to vibratory motion of the engine. It has conventionally been proposed to install an electromagnetic force motor, or "shaker" in parallel with the spring and damper of each mount. An accelerometer mounted on the support in the vincity of the mount, supplies a signal to a controller which operates the "shaker" to produce an output force of like magnitude but 180 degrees out of phase with respect to the sum of the vibration forces transmitted through the spring and damper, such that the net force transmitted through the suspension is substantially reduced to zero.
A, further conventional system for actively controlling the vibrational forces exerted from an engine is disclosed in an article entitled "Open-Loop Versus Closed-Loop Control for Hydraulic Engine Mounts" by Graf et al and published in S.A.E. publication number 880075, published in 1988. The hydraulic mount system includes a rubber structure capped by thin metal plates at both ends. A metal bushing carries the load of the engine and is situated between two compliant fluid reservoirs. Motion of the metal bushing is controlled via a close-coupled servo-valve to deliver pressurized hydraulic fluid alternately to opposing reservoirs within the mount. This permits an active mount to impart either an attractive or repulsive force between the power train and chassis.
Recently there have been attempts to drive a bidirectional hydraulic mount directly with a controllable pump mechanism. Such an approach avoids the need for a separate bidirectional servo-valve and pump. Such an approach was described in published PCT application WO89/05930 published Jun. 29, 1989. In this approach the pump was physically separated from the mount and hydraulically connected thereto with hydraulic lines.
The above approach has not proved particularly successful as it was not possible to transmit enough force from the pump to the mount. Additionally, separate mounts and pumps were necessary.
In the system of this PCT application, a spring and damper are arranged in parallel with one another between the masses. The damper has first and second fluid-containing chambers, continuously communicating with one another through a restricted orifice. The PCT application attempts to use the pump to create a net pressure differential across the orifice to reduce the dynamic force attributable to such relative motion between the masses and transmitted through the spring-and-damper. The pump is arranged such that a attempt is made to substantially cancel the dynamic force transmitted through both of the spring-and-damper attributable to such relative motion between the masses.
However, though the conventional systems previously mentioned idealistically appear to provide systems in which a fluid displacement generating device produces a desired pressure drop across an orifice of such polarity, magnitude and phase, so as to oppose and reduce certain forces transmitted through a spring and damper combination, they do not, in actuality, operate in such a manner. Power inefficiencies occur due to pumping the hydraulic fluid, and further losses exist in producing proper forces to actively control and account for machine vibration. Further, by utilizing a fluid displacement generating device, or similar type electrohydraulic servovalve, a bulky package is produced which is difficult to implement to provide a practical actively-controlled machine mount. Still further, inefficiencies result due to "bulging" along nonworking mount axes. "Bulging" is an action which exists such that the walls in particular chambers do not allow for accurate force transmission via the liquid flowing through the particular chamber.
The following analysis indicates the issue regarding pumping fluid in this application. The following description and analysis of fluid pumping will be illustrated with, regard to FIG. 2(a) and 2(b). FIG. 2(a) illustrates a conventional system, showing a particular length of tubing through which fluid must flow. A piston attached to a drive motor can provide a force (F1 as shown in FIG. 2(b)) to push a column of fluid at a particular frequency. The column of fluid has a mass M.sub.1 as shown in FIG. 2b). The force, F.sub.2 as shown in FIG. 2(b) is the remaining force available to displace a working "bushing" of an active mount. This bushing, for example, can be seen with regard to 3 of FIG. 1. FIG. 3 is a representative curve of the characteristics of a whole series of plots (not shown) showing the effect of the frequency function on causing increasing loss of available force. Clearly, the effect of this loss is not as important at "low" frequencies as it is as "high" frequencies.